The present invention relates to a stencil printer and, more particularly, to a stencil printer capable adjusting the density of an image to be printed on a recording medium.
Some different methods are available with a stencil printer for adjusting the density of an image to be printed on a paper or similar recording medium. For example, there may be adjusted the amount of ink to be fed to an ink roller, the pressure to be executed by the ink roller on a print drum, or a pressure to be executed by a press roller on the print drum. Among them, the adjustment of the amount of ink to be fed to the ink roller can be done by adjusting the gap between the ink roller and a doctor roller. However, this kind of adjustment is difficult because the gap between the above rollers is small and because the gap must be uniformly adjusted over the entire length of the rollers. The adjustment of the pressure to be executed by the ink roller on the print drum is also difficult because the pressure must be adjusted uniformly over the entire length of the ink roller. By contrast, the pressure to be executed by the press roller on the print drum can be comparatively easily adjusted because the pressure should only be variable. This kind of scheme is taught in, e.g., Japanese Patent Laid-Open Publication No. 2-151473.
The problem with a conventional print drum formed with pores in a low density is that the amount of ink to infiltrate into a paper greatly depends on whether or not an image aligns with the pores. It is therefore likely that blurring, offset and other defects occur due to local increase in the amount of ink even when the average image density is low. A current trend in the stencil printers art is toward a print drum formed with pores of small diameter in a high density in order to reduce the offset and blurring and to increase the average image density. While this kind of print drum is usually produced by etching, the minimum diameter of the pores is substantially determined by the thickness of a sheet constituting the drum. Therefore, should the diameter of the pores be excessively reduced in order to increase the density of the pores, the drum would be reduced in thickness and therefore rigidity.
Assume that the adjustment of the pressure of the press roller stated previously is applied to the drum whose rigidity is low. Then, when the press roller presses the drum, the drum yields easily and prevents the expected pressure to act on an ink layer, i.e., the pressure necessary for printing from being set up. As a result, the amount of ink oozing out from the drum is too small to produce a high quality printing. Should the pressure to act on the drum be increased in order to increase the amount of ink, the drum would greatly deform and would cause members supporting the drum to break. In addition, such a high pressure would lower the durability of the drum and would aggravate noise ascribable to the contact of the pressing member with the drum. Moreover, after the drum has been pressed by the pressing member, it resiliently restores its original position. If the drum is excessively deformed by the high pressure, it is apt to fail to restore. Once the drum deforms, the deformed portion of the drum appears on a printing as a defective image. The drum should be bodily replaced if its deformation exceeds an allowable limit. Therefore, when the drum has low rigidity, there is a demand for a method capable of adjusting image density without varying the mechanical pressure to act on the drum of the printer.
Varying the print speed is another prior art implementation for adjusting the image density. This kind of approach, however, varies the time when a printing is to be output from the printer, and thereby lowers productivity.